1. Field of the Invention
The present invention relates to a structure of a semiconductor device including a two-dimensional quantum well structure formed on a diffraction grating so as to realize a semiconductor laser having a low threshold current density, a high-speed transistor, and the like, formation of a diffraction grating meeting this object, and a method of manufacturing a semiconductor crystal on the diffraction grating. The present invention is applied to the manufacture of a semiconductor element having high-performance electronic or optical characteristics.
2. Related Background Art
Conventionally, extensive studies have been made about a semiconductor structure in which a diffraction grating is formed on a semiconductor substrate, and a layer having a second periodic structure shifted from the phase of the periodic structure of the substrate is grown on the diffraction grating.
A diffraction grating is used in various optical circuit elements such as a filter, an optical coupler, a distributed feedback (DFB) laser, a distributed Bragg reflection (DBR) laser, and the like in the field of optical electronics. In particular, a diffraction grating formed for a wavelength control or wavelength variable semiconductor laser represented by the DFB or DBR laser is used as a laser resonator. In addition, the period, shape, and depth of the diffraction grating are important factors for determining laser characteristics (an oscillation threshold, a coupling coefficient, and the like), and to form a high-precision diffraction grating in such a laser with high controllability is an important subject in this technical field.
Conventionally, a diffraction grating is formed by a two-step method including formation of a grating-shaped photoresist mask, and etching. However, since the period of a diffraction grating used in the optical electronics field is as fine as 0.1 to 1.0 .mu.m, the conventional photolithography technique cannot be applied to the formation of the photoresist mask. For this reason, a holographic exposure method is normally adopted. This exposure method uses an interference of laser light, and its process will be explained below with reference to FIGS. 1A to 1D.
As shown in FIG. 1A, a photoresist 1-22 is coated on a substrate 1-21. Two laser beams 1-23 and 1-24 which are sufficiently parallel light beams are radiated on the photoresist layer from two directions (in FIG. 1A, two directions each forming an angle .theta. with respect to the perpendicular) to form interference fringes, thereby periodically exposing the photoresist layer. The exposed photoresist layer is developed and baked, thereby forming a grating-shaped photoresist mask 1-25, as shown in FIG. 1B. Then, etching (wet or dry etching) using the photoresist mask as an etching mask is performed, as shown in FIG. 1C, and thereafter, the etching mask is removed, thereby transferring a periodic structure (diffraction grating) 1-26 to the substrate, as shown in FIG. 1D.
In the holographic interference exposure method, if the incident angle of the two laser beams is represented by .theta., and the wavelength of the laser beam is represented by .lambda., a grating interval .LAMBDA. (see FIG. 1B) which can be formed is given by .LAMBDA.=.lambda./(2sin.theta.). As an exposure laser, an Ar laser (.lambda.=351 nm) or an He--Cd laser (.lambda.=325 nm) is suitably used. In order to form a mask having .LAMBDA..ltoreq.0.25 .mu.m, the He--Cd laser is used. On the other hand, the oscillation wavelength .lambda. of a DFB or DBR semiconductor laser can be expressed by .lambda.=.lambda..sub.0 /N.sub.eff =2.LAMBDA./m (m=1, 2, . . . ) (where .lambda..sub.0 /N.sub.eff is the Bragg wavelength in a medium, N.sub.eff is the equivalent refractive index of the medium, and .LAMBDA. is the period of a diffraction grating in this semiconductor laser). m=1 as an integer means 1st-order diffraction, and m=2 means 2nd-order diffraction.
In such a semiconductor laser, for example, in a short-wavelength laser having a GaAs layer as an active layer, the period .LAMBDA. of the diffraction grating becomes as small as about 115 nm from the above equation (in the case utilizing 1st-order diffraction) if the oscillation wavelength is 0.8 .mu.m. Even when mth-order diffraction is utilized, the period .LAMBDA. can only be increased to m times of 115 nm. Therefore, in order to decrease the wavelength of an exposure laser beam in the formation of the diffraction grating, the 1st-order diffraction grating cannot be formed in air even when the He--Cd laser having a wavelength .lambda.=325 nm is used.
Thus, as a method of manufacturing a 1st-order diffraction grating (period .LAMBDA.=130 nm or less) of a GaAs-based short-wavelength DFB or DBR semiconductor laser, the following three methods are known.
As the first method, a sample is dipped in a high-refractive index medium, and interference exposure is performed therein, thereby shortening the period of the diffraction grating by the refractive index of the medium. In the high-refractive index medium, xylene with small light absorption is used. As a similar method, a triangular or rectangular prism may be placed on a photoresist film via a refractive index matching oil, and two exposure light beams may be caused to become incident from the two sides of this prism.
In the second method, the period of the diffraction grating obtained by a two-beam interference exposure method is halved by another treatment. As illustrated in FIG. 2A, a plurality of grooves (diffraction grating) 2-27a are formed on a substrate 2-27, and an etching mask material (photoresist) 2-28 is formed on the entire surface of the substrate. Thereafter, the etching surface is exposed by exposure and development, as shown in FIG. 2B, and etching is performed, as shown in FIG. 2C, thereby obtaining a diffraction grating having a period half that of the grooves 2-27a, as shown in FIG. 2D.
As the third method, as illustrated in FIGS. 3A to 3D, a photoresist mask 3-29 is formed on a substrate 3-30 by a two-beam interference exposure method, as shown in FIG. 3A, and an SiN.sub.x film 3-31 is grown on the mask by electron cyclotron resonance (ECR)-chemical vapor deposition (CVD), as shown in FIG. 3B. The etching time is adjusted, thereby forming resist and SiN.sub.x etching masks 3-29 and 3-32, as shown in FIG. 3C. Etching is performed using these masks, thereby forming a diffraction grating 3-33 having a period half that of the initially formed photoresist mask, as shown in FIG. 3D.
The manufacture of a wafer multi-layered structure on a manufactured diffraction grating is a very important technique for the sake of improving the semiconductor technique, and the threshold current density of a laser.
A semiconductor device using a superlattice structure has received a lot of attention since it realizes a semiconductor laser having a lower threshold current and a transistor capable of performing a higher-speed operation as compared to conventional ones. As the superlattice structure under examination, a one-dimensional quantum well structure (quantum thin film) is the main stream, and a semiconductor laser utilizing this structure is described in the following reference:
Tsang, W. T.: "Extremely Low Threshold (Alga)As Modified Multi-Quantum Well Heterostructure Laser Grown by Molecular Beam Epitaxy" Appl. Phys. Lett., 39, p786-788 (1981)
In contrast to the one-dimensional quantum well structure, if the number of dimensions is increased like a two-dimensional quantum well structure (quantum wire), a three-dimensional quantum well structure (quantum box), and the like, it is presumed that devices having more excellent features can be manufactured.
FIGS. 4A, 4B, and 4C are perspective views showing the principles of the one-, two-, and three-dimensional quantum well structures, respectively, and FIGS. 5A, 5B, and 5C are graphs each showing the relationship between the density of states and energy shown in FIGS. 4A, 4B, and 4C, respectively.
Although the density of states of the one-, two-, or three-dimensional structure in a bulk crystal state is defined by a parabola, as indicated by a dotted curve in FIG. 5A, the curve of the density of states is changed to a staircase shape, sawtooth shape, and pulse train shape respectively in the one-, two-, and three-dimensional quantum well structures. Since it is presumed that a light absorption or light-emission state sequentially changes with such a change in density of states, a semiconductor laser having an extremely low threshold is expected. Furthermore, in the two-dimensional quantum well structure, it is presumed that the electron mobility is increased due to a simplified scattering mechanism, and such a structure is also important in view of electronic devices.
In a quantum wire laser using the two-dimensional quantum well structure or a quantum box laser using the three-dimensional quantum well structure, the following effects are expected from the above-mentioned features:
(1) low threshold laser; PA1 (2) small temperature dependency of threshold current; PA1 (3) improvement of high-speed modulation due to increase in relaxation vibration resonance frequency; and PA1 (4) narrow oscillation spectrum line width. PA1 1. to avoid generation of crystal defects and mixing of impurities; PA1 2. to allow formation of a wire in a shape and dimensions large enough to obtain a sufficient quantum confinement effect; PA1 3. to form a wire pattern with high reproducibility without being influenced by a fluctuation upon atomic diffusion; and PA1 4. to allow a high-density arrangement of quantum wires with a high degree of freedom. PA1 a substrate which consists of a semiconductor having one of sphalerite and diamond crystal structures, and has, in at least a portion thereof, a plane orientation inclined at 0.5.degree. to 15.degree. with respect to one of {111} and {110} planes indicated by Miller indices; PA1 a first semiconductor layer which is formed on the substrate, and has a sawtooth-shaped first periodic structure consisting of one of the {111} and {110} planes indicated by the Miller indices and at least one plane indicated by another index; and PA1 a second semiconductor layer which is formed on the first semiconductor layer, and has a second periodic structure having a phase shifted from a phase of the first periodic structure. PA1 preparing a substrate which consists of a semiconductor having one of sphalerite and diamond crystal structures, and has, in at least a portion thereof, a plane orientation inclined at 0.5.degree. to 15.degree. with respect to one of {111} and {110} planes indicated by Miller indices; PA1 forming, on the substrate, a first semiconductor layer which has a sawtooth-shaped first periodic structure consisting of one of the {111} and {110} planes indicated by the Miller indices and at least one plane indicated by another index; and PA1 forming, on the first semiconductor layer, a second semiconductor layer which has a second periodic structure having a phase shifted from a phase of the first periodic structure. PA1 preparing a substrate which consists of a semiconductor having one of sphalerite and diamond crystal structures, and has, in at least a portion thereof, a plane orientation inclined at 0.5.degree. to 15.degree. with respect to one of {111} and {110} planes indicated by Miller indices; PA1 forming a diffraction grating on a surface of the substrate; and PA1 forming a semiconductor layer on the diffraction grating using one of solid-state source molecular beam epitaxy and gas source molecular beam epitaxy, PA1 wherein a direction of a central line of a molecular beam of at least one molecule type forms an angle of 10.degree. to 80.degree. with a normal to a plane orientation of the substrate.
As described above, although increase in the number of dimensions of quantization has various merits, its manufacturing method is still under development. As a manufacturing method under examination, a method shown in FIGS. 6A to 6D (disclosed in Japanese Laid-Open Patent Application No. 63-94615) is the main stream. The principle of the manufacturing method will be described below with reference to FIGS. 6A to 6D and FIG. 7.
Referring to FIG. 6A, a substrate having a (100) plane on its surface is inclined at several degrees toward a (011) direction. The inclined plane is defined by combining a (100) plane 131 and a (011) plane 133. Assume that an AlAs molecule 132 flies onto a crystal face (100). At this time, since the AlAs molecule 132 is unstable on the (100) plane, it is re-evaporated or moves on the (100) plane. When the molecule reaches a step with the (011) plane 133, the AlAs molecule 132 is absorbed by this step, and forms a crystal face. As shown in FIG. 6B, more AlAs molecules 132 are absorbed by this step in turn, and become a crystal. As shown in FIG. 6C, supply of the AlAs molecules 132 is stopped to form an AlAs layer halfway on the (100) plane. Then, when molecules to be supplied are changed to GaAs molecules 134, crystal growth is performed based on the same principle as described above, and GaAs and AlAs regions are formed in the horizontal direction, as shown in FIG. 6D. A film thickness h 135 of each layer corresponds to one atomic layer. When the growth processes shown in FIGS. 6C and 6D are repeated, a crystal structure in which GaAs layers 141 and AlAs layers 142 extend in the vertical direction, as shown in FIG. 7, can be obtained. In this manner, when an inclined substrate is used, and the molecular species to be supplied is changed, quantum wires can be manufactured.
A crystal growth method will be described in detail below.
It is expected that new electronic or optical characteristics different from those of a semiconductor crystal in a bulk state appear in a quantum wire in which electrons and holes as carriers in a semiconductor are confined in a one-dimensional narrow space, or in a two-dimensional superlattice which is constituted by periodically arranging the quantum wires. For example, when the quantum wire is used in an active layer of a semiconductor laser, the threshold current of laser oscillation can be lowered, and a sharp light emission spectrum which is stable against a change in temperature can be obtained. Since the electron mobility in the quantum wire is as high as about 10.sup.7 to 10.sup.8 cm/V.sec, applications to a field effect transistor (FET) having the same high-speed characteristics as those of a high electron mobility transistor (HEMT) and a unique function element are expected.
For these reasons, attempts have been made to manufacture the quantum wire and the two-dimensional superlattice. Quantum confinement in the one-dimensional direction can be achieved with a high precision by thin film formation techniques such as an MBE method. In contrast to this, in order to manufacture a quantum wire in which carriers are confined in the two-dimensional directions, certain means for confining a region, corresponding to a well, of an energy band in a barrier region by modulating the energy band in the planar direction in addition to the direction of film thickness is required. Methods proposed and examined as means for manufacturing the quantum wire or the two-dimensional superlattice are classified into the following three types.
(Prior Art 1)
A method of directly utilizing micropatterning techniques such as lithography, etching, and the like as means for forming an interface for confining a GaAs well region in the planar direction.
(Prior Art 2)
A GaAs well layer and an AlGaAs barrier layer are grown on a GaAs (001) OFF substrate pre-formed with a three-dimensional pattern using the metal-organic vapor phase epitaxy (MOVPE) method. A method of forming a quantum wire on the bottom surface of a V groove or on a sawtooth-shaped step portion by utilizing a difference between growth rates depending on crystal orientations in this case.
(Prior Art 3)
If growth conditions are suitably selected upon MBE growth on a GaAs substrate having a surface orientation slightly inclined from a -(001) plane, crystal growth progresses by movement of atomic steps. A method of forming a quantum wire structure by alternately growing GaAs and AlGaAs within a time shorter than a time interval for forming one atomic layer utilizes this fact.
The above-mentioned manufacturing methods of the 1st-order diffraction grating (period .LAMBDA.=130 nm or less) for a short-wavelength (.lambda.=0.8 .mu.m) DFB or DBR semiconductor laser suffer from the following problems.
In the first method, the light wavefront of an exposure beam is disturbed by scattering or multiple reflections caused by a liquid or its container, or is easily influenced by a vibration of the apparatus and a fluctuation of air, thus impairing the precision of the diffraction grating.
In the second method, it is difficult to control the exposure/development conditions after the photoresist film is formed as an etching mask on the entire surface, and a variation occurs in a single plane, thus impairing the precision of the diffraction grating. When wet etching is used in the second etching process, asymmetry of the diffraction grating shape occurs.
In the third method, it is difficult to control the etching conditions of the SiN.sub.x film on the resist and the SiN.sub.x film on the substrate. If dry etching is used in etching, since the etching mask consists of two different materials, i.e., the resist and the SiN.sub.x film, these materials have different resistances against an etching gas, resulting in asymmetry of a diffraction grating shape, and low precision of the depth and dimensions.
The above-mentioned conventional manufacturing method of the two-dimensional quantum well structure utilizes the periodicity of AlAs or GaAs. However, the AlAs and GaAs layers do not always exhibit constant periodicity, and the widths of the GaAs and AlAs layers often vary, as indicated by 141 and 142 in FIG. 7. In this case, the quantum wires of the manufactured two-dimensional quantum well structure vary, thus considerably impairing the quantum level and density of states.
The present invention is to provide a method of manufacturing a laser having two-dimensional quantum wires with high reproducibility without causing the above-mentioned problems.
The conventional manufacturing method of the quantum wire suffers some problems.
In Prior Art 1, it is difficult to avoid generation of crystal defects caused by micropatterning or the influence of mixing of impurities. Furthermore, it is almost impossible to realize a two-dimensional superlattice in which quantum wires are arranged at a high density by this method.
The method of Prior Art 2 is superior to that of Prior Art 1 in that there is almost no fear of generation of crystal defects and mixing of impurities since the quantum wire structure is formed in a single growth process. However, the problem of Prior Art 2 is insufficient confinement of carriers due to a large limitation on the dimensions and shape of the wires which can be manufactured. For this reason, it is difficult to manufacture the two-dimensional superlattice.
The method of Prior Art 3 allows a high-density arrangement of quantum wires in principle. However, this technique is essentially easily influenced by a statistic variation or a fluctuation caused in a surface diffusion process of atoms. For this reason, no high-quality two-dimensional superlattice has been manufactured yet.
Objects to be attained by the present invention for the purpose of overcoming the conventional problems are the following four items:
On the other hand, a wavelength-stabilized laser has been enthusiastically studied as a main constituting element of an optical communication device. As a typical structure of this laser, a DFB laser is known. The DFB laser will be described below with reference to FIGS. 8A to 8D. Referring to FIGS. 8A to 8D, an n-GaAs substrate 340 is used. A 0.5-.mu.m thick Si-doped GaAs layer 341 is stacked on the substrate 340, a 1.5-.mu.m thick Si-doped AlGaAs lower cladding layer 342 is stacked, and then, a 0.2-.mu.m thick GRIN layer 343 is stacked. An undopad GaAs active layer 344 has a thickness of 6 .mu.m. After a 0.2-.mu.m thick upper GRIN layer 345 is stacked on the active layer 344, growth is temporarily stopped (FIG. 8A).
After the growth is stopped, a grating 346 is formed on the upper GRIN layer 345 (FIG. 8B). The grating 346 has a pitch of 240 nm, and is formed by an interference exposure method utilizing He--Cd (FIGS. 1A to 1D). An upper layer is stacked on the grating 346 formed in this manner. Referring to FIG. 8C, a 0.3-.mu.m thick Be-doped AlGaAs light guide layer 347 is formed on the grating 346, a 1.2-.mu.m thick Be-doped AlGaAs cladding layer 348 is grown on the light guide layer 347, and a 0.5-.mu.m thick Be-doped GaAs capping layer 349 is formed on the cladding layer 348.
In order to attain current constriction, a ridge 350 is formed in the formed multi-layered films (FIG. 8D). As a result, a current is concentrated on the lower portion of the ridge 350, as indicated by arrows 351. In this manner, a light current confinement structure is formed, and a wavelength-stabilized laser is manufactured.
However, as can be understood from the above description, this method includes four major processes. In particular, current confinement and light confinement are attained after growth, resulting in complicated processes.
As described above, it is difficult for the conventional method to manufacture a semiconductor device having at least one of a current constriction structure and a quantum wire structure such as a laser having two-dimensional quantum wires, a wavelength-stabilized layer, and the like in a small number of processes with high reproducibility.
More specifically, it is difficult to manufacture a laser having two-dimensional quantum wires with high reproducibility to have a current constriction effect. Also, a wavelength-stabilized laser requires complicated processes, and for this reason, the yield, in particular, the yield in formation of the ridge 350 (FIG. 8D), is impaired.